GENERAL PHYSIOLOGY OF NERVE-TISSUE 



97 



cell, by the physiologic or slightly toxic action of various drugs, e.g., 

 curara, adrenalin, atropin, pilocarpin, nicotin, etc. 



The terminal branches of the axons of the preganglionic neurons and 

 the dendrites of the cells of the post-ganglionic neurons form what has 

 been called a synapse. Though the terminal fibers and the dendrites are 

 in close histologic relation they are not in absolute contact, but they too 

 are associated through the intervention of a receptor material. The same 

 fact it is believed holds true also in the central nerve system wherever a 

 synapse between terminal axonic branches and dendrites exist. 



The afferent nerves as they approach their ultimate terminations 

 un-dergo similar changes. The end-tufts become associated, with special- 

 ized end-organs in the skin, tongue, nose, eye, ear, muscles and tendons. 



(A consideration of these end-organs will be found in the chapters de- 

 voted to the organs of which they form a part.) 



Development and Nutrition of Nerves. The efferent nerve-fibers, 

 which constitute some of the cranial nerves and all the ventral roots of the 

 spinal nerves, have their origin in cells located in the gray matter beneath 

 the aqueduct of Sylvius, beneath the floor of the fourth ventricle, and in the 

 ventral horns of the gray matte.r of the spinal cord. These cells are the 

 modified descendants of independent, oval, pear-shaped cells the neuro- 

 blasts which migrate from the medullary tube. As they approach the 

 surface of the cord their axons are directed toward the ventral surface, which 

 eventually they pierce. Emerging 

 from the cord, the axons continue to 

 grow, and become invested with the 

 myelin sheath and neurilemma, thus 

 constituting the ventral roots. (Fig. 47.) 



The afferent nerve-fibers, which 

 constitute some of the cranial nerves 

 and all the dorsal roots of the spinal 

 nerves, develop outside of the central 

 nerve system and only subsequently 

 become connected with it. (See Fig. 

 47.) At the time of the closure of 

 the medullary tube a band or ridge of 

 epithelial tissue develops near the dor- 

 sal surface, which, becoming seg- 

 mented, moves outward and forms 

 the rudimentary spinal ganglia. The 

 cells in this situation develop two 

 axons, one from each end of the cell, 

 which pass in opposite directions, one 

 toward the spinal cord, the other toward the periphery. In the adult 

 condition the two axons shift their position, unite, and form a T-shaped 

 process, after which a division into two branches again takes place. In 

 the ganglia of all the sensori-cranial and sensori-spinal nerves the cells 

 have this histologic peculiarity. 



The efferent fibers are therefore to be regarded as outgrowths from the 

 nerve-cells in the ventral horns of the gray matter, and serve to bring the 

 cells into anatomic -and physiologic relationship directly with the skeletal 



Pteterwr 



FIG. 47. DIAGRAM SHOWING THE MODE 

 OF ORIGIN OF THE VENTRAL AND DORSAL 

 ROOTS. (Edinger, after His.} 



